Tensioners are well known in the field of marine pipelaying, often having the task of supporting the weight of the launched pipeline. Tensioners are used for many pipelaying techniques, such as S-lay, Reel lay, J-lay. Tensioner having a large “load holding capacity” and in the field capacities of tens of tonnes, or even well over 100 tonnes, for a tensioner are not uncommon. An example of a tensioner is disclosed in U.S. Pat. No. 6,394,445 (Itrec).
In a known embodiment a marine pipeline installation tensioner comprises a tensioner frame and multiple track units mounted in said frame. The number of track units is normally adapted to the capacity and/or pipeline type and common tensioner designs include two, three or four track units (or possibly a variable number of track units). Each track unit includes a chassis and an endless track, with support rollers or other bearing means being mounted on the chassis for supporting the endless track, and one more track drive/brake members for effecting drive and/or braking motion of the track. The track is adapted to engage on the exterior of the pipeline, commonly as the track has friction elements, usually friction pads, engaging on said pipeline exterior.
It is known to arrange each track unit mobile within the associated frame so that the “gap” or “pipeline passage” between the tracks can be adapted to different pipeline diameters and possible to allow for an “widely opened” position of the track units wherein a large item (e.g. an accessory fitted in or on the pipeline, such as a pipeline connector) can pass through the tensioner.
In U.S. Pat. No. 6,394,445 it is disclosed to provide hydraulic cylinders between the frame of the tensioner and each track unit to create the “squeeze pressure” between the track and the pipeline exterior needed to hold the pipeline and absorb the pipeline load on the basis of friction and also to allow for displacement of the track unit to adapt to the pipeline diameter. It will be understood that the “squeeze pressure” needed between the pipeline and the tracks is very significant as the resultant frictional force might need to be several tens of tonnes, or even well over one hundred tonnes. The desire for very large pipeline weight support capacity of the tensioner e.g. stems from the desire to develop oil and gas fields in deepwater, where water depth (and thus length of launched pipeline supported by the tensioner, or multiple tensioners in series) might be more than 500 meters, or even over 1000 meters.
The pipelines to be launched with a pipelaying vessel equipped with a marine pipelaying system including one or more tensioners can vary significantly, i.e. with respect to pipeline properties (rigid pipeline or flexible pipeline, coated or non-coated, reeled or non-reeled, etc). Therefore the need exists for tensioners to be able to handle a wide range of different pipelines (possibly fitted with accessories having a larger cross-section than the pipeline itself).
The prior art tensioners have proven to be limited in respect of their range of application.
For instance a problem is that a pipeline may include a section having a non-uniform diameter, e.g. a lightly tapered section or other “minor” variation of pipeline diameter. As common tensioners are build to keep the tracks exactly parallel to the pipeline passage axis, the presence of such a lightly tapered section or other “variance” in pipeline diameter may result in overloading of the pipeline by the tracks.